The present invention relates to methods for setting the loop gain in a servo loop of a tracking servo device of an optical disc player.
What is essential for an optical disc player for playing an information storage disc (referred to as disc, hereinafter) such as a video disc, a digital audio disc and the like, is a tracking servo loop for controlling a laser light spot, which serves as an information reading spot of a pick-up, to make it precisely track a storage track irrespective of any eccentricity of the disc and the like. Some known examples of tracking servo-loops operate according to the so-called 3-beam method, in which there is provided an information reading main beam and two tracking error detecting auxiliary beams. The auxiliary beams are arranged on both sides of the main beam such that the line on which the three beams fall is placed at a predetermined offset angle with a tangent line of a track. An error signal is generated on the basis of the difference between the luminous quantities of the two auxiliary beams reflected from the information storage surface of a disc.
In such a tracking servo loop, it sometimes occurs thay the loop gain varies from disc to disc due to a variation in the reflectivity of a disc being played. The fluctuation in loop gain leads to a decline in tracking performance with respect to focusing error, which will not provide stabilized servo operation.
For this reason, it has been traditional to set the loop gain for each disc by detecting the peak-to-peak values (differences between positive and negative wave heights) fo a tracking error signal, which are generated when the tracking loop is open and the disc is stationary, and by setting the loop gain accordingly.
However, if the disc being played has any eccentricity, several problems result with the above method. At stationary or nearly stationary revolving speeds, eccentricity of a disc will cause the relative speed of the laser light spot in crossing a track to increase, causing the frequency of the tracking error signal to rise. Additionally, because a low pass filter (LPF) circuit is placed before the input of an A-D converter (for converting the error signal into digital data) to attenuate undesirable frequency components such as noise, the wave height of the tracking error signal drops as shonw in FIG. 7(A). It thus becomes impossible to detect precisely the peak-to-peak values of the tracking error signal, and consequently precise loop-again setting becomes impossible.